Ground Based Augmentation System (GBAS)

GBAS Overview

GBAS Platform

Initial research and development efforts have shown that navigation systems relying on GNSS ground-based augmentation systems (GBAS) can be considered as a promising solution for approach and landing in all weather and visibility conditions up to CAT III. In this context, the Institute of Communications and Navigation at DLR already created a hardware / software GBAS demonstration platform with full CAT I approach functionality, as part of DLR’s progressive research into CAT III capable GNSS landing systems. The next research milestone at DLR is to build a demonstration for a CAT II / III capable GBAS.

Ground architecture:

The ground subsystems located at DLR’s research airport in Braunschweig are composed of three multi frequency multi constellation receivers providing real time observations to a GBAS central processing facility. Here, corrections are computed and integrity monitoring algorithms evaluate the quality of the data. Corrections and integrity information can be included in the standard message types (ICAO SARPS and EUROCAE ED-114) and broadcast to the aircraft during approach phase.

This GBAS platform offers the possibility of integrating a software-based receiver as well as implementing and testing improved signal quality monitoring, multipath and interference mitigation/monitoring algorithms.

In addition to the core processors constituting the GBAS ground subsystem, it is necessary to implement additional types of monitors to augment the robustness of the GBAS solution for Safety of Life applications. The most important of these are ephemeris, ionosphere and signal quality monitors.

Airborne architecture:

The airborne receiver applies pseudorange and range-rate corrections as broadcast from the ground and computes a Position, Velocity and Time (PVT) solution. In parallel to the PVT, integrity information is processed onboard using the error bounds provided by the ground processing facility and airborne monitors. Both, the airborne raw data and the processing output are then provided to the pilot and broadcast via TCP/IP to the GBAS demonstration terminal.

To meet the most stringent requirements for a civil aircraft navigation system for approach and landing under zero visibility (CAT IIIc), additional airborne monitors need to be implemented. Under the single-frequency GPS CAT III scenario, meeting the GBAS Approach Service Type D (GAST-D ) requirements implies the necessity of monitoring the temporal ionosphere gradients. Therefore, a second 30 s smoothing filter needs to be implemented in addition to the standard 100 s carrier smoothing. Additionally, novel airborne integrity monitoring algorithms are currently under development for implementation in a future airborne receiver.

GBAS Board Monitor

GBAS Ground Monitor

GBAS performance demonstration:

The demonstration graphical display is located at DLR Oberpfaffenhofen. Currently, the processing power is provided by an eight-core computer running Linux and a four-monitor display is used for the output. The GUI provides real time information from ground and airborne subsystems, with the following main parameters being part of the displayed output:

PVT solution including protection levels

Dilution Of Precision (DOP) values

Range and range rate corrections

B-Values for ground receiver fault detection

P-Values as a measure of ephemeris decorrelation

All parameters are then stored on disk for post-processing validation. Also under development is an interface to the DLR MASTER signal generator for post-processing hardware validation.

Future GBAS navigation concepts:

Our current GBAS demonstration platform will be used as a development platform for future concepts. Important necessary upgrades have been already identified: